[Rpm-ecosystem] varlink for libdnf discussion

[Colin Walters]

See also this post about varlink for podman:
http://www.projectatomic.io/blog/2018/05/podman-varlink/

To emphasize though I'm not 100% sure about this path; I have
a lot of experience with language bindings and shared libraries,
but none with varlink.  

That said I may end up experimenting with this at least at
the rpm-ostree level; I've been wanting to try to move
all of the transactions from a thread in the daemon to a subprocess
in order to fix things like:
https://github.com/projectatomic/rpm-ostree/issues/897
I could do that inside rpm-ostree using varlink, and the API
for that would probably look a lot like what the core of a libdnf
varlink API would look like.  (Though this ramps up the complexity
in some areas as we already have a stable DBus API so many
events would go worker process -> varlink -> DBus -> client)